The present invention relates to tire condition monitoring apparatuses for monitoring the condition of the tires of a vehicle, and, more particularly, to structures of signals transmitted by transmitters, which are attached to the tires.
Conventionally, a wireless type tire condition monitoring apparatus has been used for permitting a driver of a vehicle to monitor the condition of the tires in the passenger compartment. The apparatus includes transmitters, each of which is associated with a different one of the tires and is attached to the tire, and a receiver mounted in the body frame of the vehicle. Each of the transmitters detects the condition of the associated tire, such as the tire pressure and the interior temperature of the tire, and wirelessly transmits a signal including data that indicates the detection results to the receiver. On receiving the signal, the receiver displays the condition of the tire on a display, which is located, for example, near the driver seat, as needed.
FIG. 6 illustrates the structure of the signal transmitted by each transmitter. With reference to the drawing, the signal includes successive, or first to third, data frames. Each of the data frames includes six types of data, which are synchronous data, identification code data, pressure data, temperature data, voltage data, and error detection code data. The synchronous data indicates the beginning of each data frame. The ID code data indicates a specific identification code of each transmitter. The pressure data and the temperature data respectively indicate the tire pressure and the tire interior temperature. The voltage data indicates the voltage of a battery, which is the power source of each transmitter. The error detection data permits the receiver to judge whether or not each data frame has an error. The first to third data frames are identical. In other words, each transmitter repeats transmission of the same data frame, which includes the above-described six types of data, for three consecutive times.
In each data frame, the six types of data are each configured by a binary code signal of a predetermined bit count. The bit code “0” is either one of the two types of square wave signals shown in FIG. 7(a), one cycle of which lasts 800 microseconds. The bit code “1” is either a high or low level signal shown in FIG. 7(b), one cycle of which lasts 800 microseconds. That is, the bit code “1” is configured by a so-called bi-phase signal.
In each data frame, each of the data other than the synchronous data is an 8 bit data. The data portion corresponding to the last bit of the 8 bit data is an end code indicating the completion of the 8 bit data. The end code of each 8 bit data is different from the end code of the preceding data. The synchronous data is configured by repeating the same code “0” or “1” consecutively for 8 bits or more, for example, 16 bits.
In each data frame, only the synchronous data is configured by the single code that is repeated consecutively for 8 bits or more. Thus, when the receiver receives the signal from a certain one of the transmitters, the receiver reliably recognizes the synchronous data in the signal. This enables the receiver to accurately read out the other five data, which are preceded by the synchronous data. In other words, if the synchronous data is unrecognizable, the receiver is not permitted to read out the content of the received signal.
As long as the tires are in normal condition, the transmitters transmit data at predetermined time intervals. However, when the engine of the vehicle is stopped, the receiver is operated intermittently to save the vehicle's battery. More specifically, as long as the vehicle engine is held as stopped, the operation of the receiver remains non-continuous, or the receiver is permitted to receive data from the transmitters only intermittently after predetermined waiting periods.
Thus, during the intermittent operation, it is difficult for the receiver to receive the signal of the transmitters from the beginning of the signal. That is, the receiver in the intermittent operation usually starts to receive the transmitted signal after the signal transmission has already started. However, each of the waiting periods of the intermittent operation is shorter than or equal to the time needed for each transmitter to transmit a single data frame. Further, when the receiver receives a signal from a certain one of the transmitters on resuming its operation during the intermittent operation, the receiver is held as activated until the signal reception is completed, regardless of the intermittent operation. Thus, even if the receiver is not permitted to receive the first data frame of the transmitted signal from the beginning of the first data frame, the receiver may be permitted to fully receive the second and third data from the beginning of each of the data frames. In other words, even in the intermittent operation, the receiver is permitted to complete reception of at least one data frame of the transmitted signal. This enables the receiver to obtain necessary information from the transmitted signal.
The synchronous data of each data frame only indicates the beginning of the data frame. That is, unlike the other five types of data (the 8 bit data, such as the pressure data), the synchronous data does not include any information that must be transmitted from the transmitters to the receiver. Thus, in order to shorten the data length (the total bit count) of the signal transmitted by the transmitters, it is desirable that the bit count of the synchronous data be relatively small. However, as described, the conventional synchronized data is configured by the codes of the bit count greater than or equal to the bit counts of the data other than the synchronous data. The transmitted signal thus has a relatively long data length, thus prolonging the time needed for transmitting the signal. This shortens the life of the battery incorporated in each transmitter.
Further, when the vehicle is traveling, the transmitters rotate integrally with the tires such that the orientation of each transmitter continuously changes relative to the receiver. Since a transmitting antenna of each transmitter and a receiving antenna of the receiver are both directional, the rotational angles of the tires greatly affect the reception level of the receiver with respect to the transmitted signal. That is, the signal reception level of the receiver may become lower than an acceptable level depending on the rotational angles of the tires. Thus, if each data frame has a relatively long data length and the time needed for transmitting the data frame is prolonged, the signal reception level of the receiver may become unacceptable during the reception of the data frame. In this case, it is highly likely that the receiver becomes incapable of completing reception of any data frame. Accordingly, to improve the reception reliability of the receiver, the data length of each data frame must be minimized such that the data frame can be transmitted in a minimum time. However, as described, the conventional synchronous data has a relatively large bit count, thus increasing the bit count of each data frame. The reception reliability of the receiver is thus lowered.